1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method.
2. Description of the Related Art
Conventionally, as one of resolution conversion functions, a super-resolution processing technique which generates high-resolution image data from a plurality of low-resolution image data is known. With this technique, even image data acquired by a low-resolution device can be converted into high-resolution image data (for example, see Japanese Patent Laid-Open No. 2007-151080).
However, upon using the super-resolution processing technique, a plurality of low-resolution image data which have phase shifts for sub-pixel units (a unit smaller than one pixel) have to be prepared, thus taking much time for preprocessing. For this reason, the use of the super-resolution processing technique is limited to a specific field that requires high-quality image data such as moving image data sensed by a digital video camera and still image data sensed by a digital camera (for example, see WO2004/068862).
However, recently, the field of image processing apparatuses such as a copying machine and facsimile apparatus requires high-quality image data. Hence, application of the super-resolution processing technique to this field has been contemplated.
Note that the image processing apparatuses such as a copying machine and facsimile apparatus normally read document images line-sequentially, and use a CCD line sensor, contact image sensor, and the like as reading devices.
However, in recent years, popularization of digital cameras accelerates a price reduction of CCD area image sensors, and image processing apparatuses such as a copying machine and facsimile apparatus use CCD area image sensors.
For this reason, upon application of the super-resolution processing technique to an image processing apparatus, it is desirable to assume a case in which a CCD area image sensor is used as a reading device.
The CCD area image sensor senses images for respective frames (to simultaneously acquire image data in the main scanning and sub scanning directions) normally (when it is used in, e.g., a digital video camera).
However, when super-resolution processing is executed in an image processing apparatus such as a copying machine or facsimile apparatus, the CCD area image sensor is moved in the sub scanning direction so as to efficiently acquire a plurality of phase-shifted image data.
Then, image data obtained in this way are processed for respective lines which form the CCD area image sensor in place of frames. With this processing, phase-shifted image data as many as the number of lines can be acquired by a single operation in the sub scanning direction.
At this time, when the operation range of the CCD area image sensor is limited to that within a document size, regions on a document covered by obtained image data as many as the number of lines cannot be uniformly obtained. That is, the number of regions on the document covered by the obtained image data is large on a central part of the document, but it is small on the upstream and downstream parts of the document.
Practical examples will be described below with reference to FIGS. 24 and 25. For example, assume that a size for one frame of the CCD area image sensor is defined by the main scanning direction=290 mm and the sub scanning direction=200 mm. When the CCD area image sensor is located at the uppermost stream position, a region from the top end of the document to a 200-mm position which is the same as the size for one frame in the sub scanning direction is covered by the first frame.
When the CCD area image sensor is moved by 10 mm in the sub scanning direction, a region from a 10-mm position from the uppermost stream position to a 210-mm position is covered by the second frame.
Upon comparison between these two image data (those of the first and second frames), only one image data covers the part from the uppermost stream position to the 10-mm position of the document. On the other hand, the two image data cover the part from the 10-mm position to the 200-mm position of the document (see FIG. 24).
As a result of movement of the CCD area image sensor within the range of the document size, image data as many as the number of lines are obtained, as shown in 25a in FIG. 25. That is, a part covered by a large number of image data and that covered by a small number of image data are inevitably generated (25b in FIG. 25).
In this way, when the operation range of the CCD area image sensor is limited to that within the document size, the data amounts of image data of the upstream and downstream parts in the sub scanning direction become small. Conversely, the data amount of image data increases toward the central part in the sub scanning direction.
In this case (when image data shown in 25a in FIG. 25 are used), even when the super-resolution processing is executed, image data having a desired resolution cannot be obtained.
On the other hand, in order to avoid such situation, the operation range of the CCD area image sensor may be expanded to be larger than the range of the document size. However, when the operation range of the CCD area image sensor is expanded, the image processing apparatus undesirably becomes bulky.